Solid state ion conducting devices are used in a variety of applications including fuel cells, steam electrolyzers, oxygen concentrators, and other types of electrochemical reactors. The various ion-conducting devices typically employ a plurality of solid-state electrolyte elements constructed from materials capable of conducting a specific ionic species through the element. Materials having this capability include ceramic metal oxides such as cerium oxide, hafnium oxide, zirconium oxide, thorium oxide, and bismuth oxide. Other materials such as immobilized molten electrolyte membranes, and polymeric electrolyte membranes may also be used as ion specific electrolyte materials. The electrolyte elements are typically formed as flat plates having an electrically conductive electrode material attached to one or both of the facial surfaces of the plate. Dimensions of these flat plates vary from five to twenty centimeters (cm) by 5 to 20 cm by 10 to 250 .mu.m.
Ion conducting devices typically include a plurality of electrolyte elements arranged into a stack, with interconnects separating successive elements. The interconnects, formed in a similar shape as the electrolyte elements, space the elements apart, while grooves in the surface form gas-flow channels when the interconnects are sandwiched between the elements. The gas-flow channels enable reactant gases to flow between the elements in order to come in contact with the electrodes. The interconnects also establish an electrical pathway through the stack. In solid oxide fuel cell ("SOFC") applications, electrical interconnection of the electrolyte elements increases the electrical output obtained from the stack.
Several groove or channel configurations in the facial surfaces of interconnects are commonly used. A cross-flow configuration orients the grooves on one face 90.degree. from the grooves on the other face. When cross flow interconnects are disposed between successive electrolyte elements, two sets of channels are formed between the elements extending through the stack at right angles to each other. The 90.degree. offset orients the openings to each set of channels on different sides of the stack. The main advantage to cross-flow interconnects is that they simplify the attachment of a gas manifold to the stack. A manifold for one type of reactant gas, e.g. a fuel gas, may be attached to one side of the stack, and a separate manifold for another reactant gas, e.g. air, may be attached to another side of the stack. In some ion conducting applications, a separate manifold is used to collect product gases (e.g. pure oxygen) produced during operation of the device. Co-flow, counterflow, and parallel gas-flow interconnect configurations are less common, but are sometimes used for special applications.
In applications, air is introduced to flow through the gas-flow channels in contact with the cathode covered face of the electrolyte elements. When the air contacts the cathode, an electrochemical reaction occurs which generates the specific ionic species. The ions are then conducted through the thickness of the electrolyte elements to the anode on the opposite face of the elements. A reactant fuel gas such as hydrogen, a methane containing gas, a syn-fuel, or a light hydrocarbon fuel stock, is introduced to flow through the gas channels in contact with the anode face of the electrolyte elements. The specific ion reacts with the fuel gas at the anode surface in an electrochemical combustion reaction.
For example, at the cathode of a typical fuel cell, an electrochemical reaction occurs in which an ionic species, such as O.sup.-2 from air, is formed. The O.sup.-2 ions are conducted through the electrolyte element to the anode where they react with the fuel gas to form carbon dioxide and water. Conduction of the O.sup.-2 ions through the electrolyte element normally occurs due to a difference in the partial pressure of O.sub.2 on opposite sides of the element. In current based devices, however, an electrical potential is applied across the elements to drive the reaction.
The electrochemical reaction occurring in a SOFC is shown by the following chemical equations where methane is the fuel gas:
air side: EQU 8e+20.sub.2 .fwdarw.40.sup.-2
fuel side: EQU CH.sub.4 +40.sup.-2 .fwdarw.CO.sub.2 +2H.sub.2 O+8e
If the two electrode surfaces are electrically connected, the fuel cell produces an electrical current from the passage of electrons from one electrode surface to the other. Electrical connection of all elements in the stack enables the cumulative electrical output from all the elements to be obtained. In this way, SOFC devices produce electrical energy directly from fuel gas combustion.
In an oxygen concentrator, air is introduced to the channels in contact with the cathode covered face of the elements, and pure molecular oxygen is collected from the opposite face. Other ion conducting devices function in a similar manner, but may have structural modifications, and different reactant gas requirements.
The electrochemical reactions occur when the electrolyte elements reach an operating temperature, typically 600.degree.-1000.degree. C. for ceramic oxide based fuel cells. Thermal energy from combustion of the fuel gas in a SOFC contributes to sustaining the operating temperature.
Fuel gases and air may be supplied to the electrolyte elements of a SOFC by manifolds attached over the openings to the two sets of gas-flow channels in cross-flow configured interconnects. Similarly, one manifold may be used to supply air to one set of gas-flow channels in an oxygen concentrator, and a gas collection manifold may be used to collect the pure oxygen from the other set of channels. Herein lies the main advantage to cross-flow configured interconnects. The cross flow geometry simplifies the manifolding process by orienting the openings to gas-flow channels carrying the same gas on the same side of the stack.
Internally manifolded interconnects obviate the necessity of attaching a manifold to the side of a stack of electrolyte elements and interconnects. This type of interconnect typically has a partially enclosed passageway at one or both ends of the gas-flow channels. When the interconnects are stacked together with the electrolyte elements, the passageways fit together in register to form an enclosed chamber which functions as a manifold. A gas introduced into this manifold is distributed among each of the interconnect channels in fluid communication with the plenum. In other fully internally manifolded ion conducting devices, the electrolyte plates include internal gas-flow channels and manifold chambers.
U.S. Pat. No. 4,950,562 (Yoshida et al.) discloses a fuel cell having a typical design for a cross flow interconnect. The interconnect is a flat plate constructed from an electrically conductive material, and has a plurality of grooves on each face. When interconnects of this type are arranged between successive electrolyte elements in a stack, all of the grooves in the interconnect surface extend to the edge of the stack. With this design, a gas-tight seal between the interconnect and the electrolyte element is extremely difficult to achieve because of the long, repeatedly-interrupted seam between the edge of the interconnect and the edge of the electrolyte element. At each end of every groove is a pair of corners which must be sealed against the electrolyte element. Corners are particularly difficult to seal, and much more prone to leakage than, for example, a continuous unbroken seam. It is very important to completely seal the corners at the ends of the grooves against the edge of the electrolyte elements because the gases flowing on opposite sides of the interconnect are in close proximity at the edge of the stack. If reactant gases leak from one set of channels to the other, the fuel gas may become diluted with air, whereupon the electrochemical reaction intended to occur on opposite sides of the electrolyte elements, instead, may occur in the fuel gas channels. The reaction and the products are the same, but the electron transfer of the reaction is not harnessed to provide electrical current. Failure to contain and segregate the reactant gases is a major cause of low efficiency and performance of fuel cell systems.
The difficulty in achieving a gas-tight seal at the edges of the interconnect grooves is acknowledged in U.S. Pat. Nos. 5,045,413 (Marianowski et al.) and 5,077,148 (Schora et al.). The fully internally manifolded fuel cell design of both of these patents is largely intended to overcome the problem of gas mixing, primarily at the manifold/electrolyte/interconnect interface. The electrolyte plates and separator plates ("electrolyte elements" and "interconnects" respectively as used herein) have the same configuration and are stacked together into the fuel cell stack. Each electrolyte plate and separator plate has a flattened peripheral seal to completely seal the periphery of the electrolyte/separator interface. A plurality of perforations in each electrolyte and separator plate forms internal manifold chambers within the stack.
Although the designs in Marianowski et al. and Schora et al. significantly improve the seal between the electrolyte element and the separator or interconnect, they are still subject to problems. For example, the flow pattern through the internal manifold chambers, and between the separator plates is of less than optimal uniformity since the path length that the gases must travel from the inlet to the outlet is different through each separator plate. These unequal paths result in less flow through some of the separator plates and overload of others, resulting in less than optimal efficiency. The electrolyte plate and separator plate designs are also complex, adding to the complexity of both manufacturing operations and stack assembly. The designs also add significant weight and bulk to a fuel cell stack because the internal manifold chambers are incorporated into the stack assembly.
U.S. Pat. Nos. 4,510,212 (Fraioli), 4,499,663 (Zwick et al.), and 4,476,197 (Herceg) also disclose fuel cell structures having fully internally manifolded gas-flow passageways, ostensibly to improve the efficiency and reliability of a fuel cell. However, these structures too are quite complex, and require comparatively more space than externally manifolded fuel cells.
A need exists for an interconnect which can be used more effectively in combination with a gas supply or collection manifold. Such an interconnect would provide improved sealing between the electrolyte element and the interconnect, and offer greater sealing reliability during operation. Further, the interconnect would be readily manufacturable, and easily assembled into a stack of ion conducting electrolyte elements. The interconnect should also improve the efficiency of an ion conducting device and facilitate increased fuel gas utilization.